Investment casting apparatus with a cushion around the shell mold



NOv- 18, 1969 P. 1 woRTHlNGToN 3,478,815

INVESTMENT CASTING APPARATUS WITH A CUSHION w 0% M9 m1 E7.. H2 s EN wo d me U1 0.1 RF A INVENTOR.

/ Pfff? L 20 United States Patent O 3,478,815 INVESTMENT CASTING APPARATUS WITH A CUSHION AROUND THE SHELL MOLD Peter L. Worthington, Cincinnati, Ohio, assignor to General Electric Company, a corporation of New York Filed Oct. 27, 1966, Ser. No. 589,972 Int. Cl. B22c 9/00 U.S. Cl. 164-361 6 Claims ABSTRACT OF THE DISCLOSURE Thin walled investment cast articles, subject to compressive loads during casting, are made from an improved investment casting ceramic shell mold including a surrounding protective compressive load-absorbing temperature resistant cushion for use in casting apparatus. Examples of such a cushion include bers in the form of padding, loosely held ber such as asbestos, flexible glass cloth, etc., alone or in combinations.

This invention relates to precision casting and more particularly to investment casting apparatus, ceramic shell molds and methods for making relatively large investment castings having thin wall structures.

The art of making precision ceramic shell molds for the investment casting of articles is highly developed and widely reported as are the casting techniques using such molds. Primarily such molds and techniques are used for the precision casting of relatively small articles or large articles which, if not solid, have relatively heavy wall structures.

The use of known technology to make relatively large thin walled investment castings with ceramic shell molds and known investment casting apparatus have resulted in collapse or distortion of walls of the shell mold in the area of the thin wall portions. Such collapse is due to differential expansion between the shell mold and a metal container in which the shell mold is placed prior to mold preheating and then casting. Thus, the investment casting of thin walled casings for turbine engines, for example, of or more in diameter, have resulted in unusable castings.

In normal practice of investment casting with large molds, for example, having a diameter of l5 or more, the shell mold is generally centered within a metal conrainer having a diameter larger than the shell mold. This leaves an annular space between the container and the external lateral surface of the outer walls of the shell mold. Such space is then lled with a loose granular material, generally a ceramic such as alumina, Mullite material or foundry sand. When the container including the mold is placed in a furnace for preheating, the container, being a metal, will expand faster and generally to a greater extent than will the ceramic shell mold. When this occurs, the granular material in the annular space settles into the expanded volume because coefficient of expansion of such material and that of the mold are considerably lower than that of the container.

When the container including the mold and the granular material are removed from the preheat furnace and placed into a casting chamber, the outer metallic container loses heat very rapidly, for example dropping from 1800 F. to about 1200 F. in less than a minute. However, the ceramic mold and granular material lose heat very slowly. The attendant shrinkage of the container then places a high compressive load on the granular material which in turn transmits such load to the outer lateral walls of the investment shell. With small molds and containers, this load is not severe enough to cause a detrimental distortion or fracture. However, with larger molds and larger con- 3,478,815 Patented Nov. 18, 1969 iceN tainers, for example annular molds of about 15" or better outside diameter, the load is sufficiently great to collapse the outer walls of the shell mold causing distortion or non-lill conditions or both on casting.

Therefore, a principal object of the present invention is to provide an improved ceramic shell mold and apparatus for investment casting as well as a casting method which will prevent the transmission of compressive loads as a result of differential expansion between the mold containers, granular lill material and the ceramic invest-ment shell molds.

Another object is to provide means, secured with the external surface of ceramic shell mold walls, which will absorb loads applied to it by such differential expansion.

Another object is to provide an improved investment casting method in which the load from such differential expansion is not transmitted from a mold-carrying container to walls of a ceramic shell mold.

These and other objects and advantages will be more clearly understood from the following detailed description, the examples and the drawing in which:

FIG. 1 is an isometric, fragmentary, partially sectional view of an investment casting mold in a container;

FIG. 2 is a fragmentary, sectional view of the mold of FIG. 1, after filling the container with granular ll material;

FIG. 3 is a fragmentary sectional view of a wall section of the mold of FIG. 2 after removal from the preheat furnace;

FIG. 4 is a fragmentary sectional view of the mold in FIG. 2 including the present invention after removal from the preheat furnace;

FIGS. 5 and 6 are fragmentary sectional views of the mold of FIG, 2 including other modifications of the present invention.

In its form as an improved ceramic investment casting shell mold, the mold has around external lateral wall surfaces of outer wall portions a cushion of a temperature resistant, compressible or resilient material capable of absorbing a compressive load which wouldbe applied to the mold prior to casting. In the apparatus form, the cushion is located between the granular ll material in the mold container and the external lateral wall surfaces of the mold.

In its method form of investment casting a molten material into a ceramic shell mold, the present invention comprises the Steps of making such a ceramic investment casting shell mold by known techniques, placing a cushion of a temperature resistant compressible or resilient material around external wall surfaces of outer wall portions, placing the cushioned mold in a mold container and then. placing a granular ceramic ll material between the cushion and the mold container walls.

As can be recognized from the previous brief description and -more fully understood from the following detailed information, the principal novel feature of the present invention is the location of a temperature resistant load absorbing means, generally a exible means such as compressible or resilient means, around those portions of a ceramic shell mold to which can be applied, prior to casting, compressive loads suflicient to cause distortion, collapse, fracture or other damage to outer walls of the shell mold. Therefore the term cushion as used herein means a load absorbing means resistant to preheat and casting temperatures and of a thickness, depending on the size of the mold, suicient to absorb compressive loads which would be applied to the ceramic mold. As was mentioned before, such loads can result from differential expansion between the mold, a metallic container in which the mold is placed and a granular generally ceramic material placed in spaces between the container and the mold.

Examples of materials which have been used as the cushion include woven or grouped arrangements of fibers, the meaning of which is intended to include filaments of a non-metallic material. Some specific examples are woven glass cloth or pads sometimes referred to as Fiberglas material; sheets or pads of Fiberfrax material; loose woven, felted, padded asbestos; and glass wool.

The simpler methods of applying such compressible means around or to the external surfaces of the mold, which usually includes a relatively complex arrangement of gates, risers and sprues, involves either wrapping the external lateral surfaces or packing external lateral cavities, or both, with the material to a thickness which will absorb externally applied compressive forces. However, it should be understood that the present invention is independent of the method of application or location of the cushion. It is contemplated that a suitable temperature resistant, non-metallic material Cushion could be applied as a ock by spraying with a suitable binder, such as a silicate binder, which would withstand the temperatures of the preheat and casting furnace. Pads of material may be held in place by wrapping a sheet of glass cloth around the pads in contact with the mold.

Once the compressible or resilient material is applied to or located in spaced relation with the external wall surfaces of the ceramic mold to form a cushion, the investment casting process, including heating in a preheat furnace followed by removal and placement in a casting chamber and then casting, can be conducted in the usual and well known manner.

Reference to the drawing will clarify certain features of the present invention. In FIG. l, a ceramic shell mold shown generally at in the general form of a ring is placed within metallic mold container 12 for the purpose of handling and supporting the mold during pretreatment and casting. The mold 10 is shown with a fragmentary portion of a pouring funnel 11 which feeds molten metal to the normal gates, risers and sprues which are included with such a mold but are not shown in the drawing for simplicity of viewing. The purpose of the drawing is merely to represent a normal investment casting ceramic shell mold having a thin wall structure.

Mold 1t) is designed for the casting of a ring shaped casing having end flanges which would be produced in mold chambers or spaces 14 in FIG. 2 and port means cast within Imold chamber or portion 16. Joining the anges in the final casting is a thin wall structure produced in the mold chamber or space 18 bounded by mold walls 20 shown more clearly in FIG. 2. In one case, the outside diameter of such a cast casing is about 26-30 inches with a wall thickness at the thin wall portion of about (LOS-0.08 inch.

Although the elimination of damage to any portion of the mold is important, any damage to those portions of the rnold such as wall portions 20` which define the thin wall portion of the casting is critical. For example, if the flange area is somewhat imperfect, generally there is suicient excess material to allow correction such as by grinding or machining. However, when compressive loads applied to the external lateral wall mold surfaces, as shown diagrammatically in FIGS. 2-4 as arrows 19, are sufficiently great to rupture the fragile walls 20, irreparable damage is caused to the mold as shown in FIG. 3. A casting attempted to be formed from the damaged mold has a hole as a result of the non-fill portion of the mold.

Consequently, application of a temperature resistant compressible or resilient means or cushion Z1 in FIG. 4 between external lateral wall surface 22 at the outer periphery of mold 10 and granular ceramic material 26 or along the surface of external lateral wall surface 22 as in FIGS. 5 and 6 is essential to obtain a sound casting. The material which is shown in FIG. 4 as representative of the cushion 21 are wrapped sheets of glass fiber forming a diaphragm-type cushion around thin wall portions 2G of the mold. In FIG. 5 the cushion is comprised of pads 21a of glass wool material held in place by sheets of glass cloth 2lb.

During normal operation prior to the present invention, mold 10 is placed in the container 12, as in FIG. l, on a bed of granular ceramic material of the same type later used to fill the container around the mold. Then spaces or voids between the mold and the container are filled with a loose granular ceramic material 26 as shown in FIG. 2. The purpose of packing the mold in this way primarily is to hold heat in the mold after heating in the preheat furnace prior to casting and to prevent rapid chilling of the mold and cast metal after casting has been completed.

After the mold is located in container 12 in this manner, the container including the 4mold is placed in a preheat furnace. This heats the empty mold to a temperature, such as about l800 F., so that upon subsequent casting of hot molten metal, the mold will not be unduly ther-t mally shocked to produce mold cracking. Then the container including the mold is removed from the preheat furnace and placed in a casting chamber which includes the melting and casting furnace.

Because container 12 is metallic and has a coefficient of expansion significantly greater than either the material of mold 10 or the ceramic granular material 26, during preheating the container will expand to a volume greater than will the other materials. Consequently, the space between external lateral wall surface 22 of mold 10 and the lateral walls of container 12 will expand from that at which it was when the granular material 26 was first introduced. As a result, granular material 26 will settle into the expanded space to position 25 as shown in FIG. 3 compared with phantom line 27 representing the original surface position of the granular ceramic material shown in FIG. 2.

Upon removal of the container including the mold from the preheat furnace, the container will lose heat very rapidly because the walls of container 12 are metallic, are relatively thin, have a large surface area and have a greater coefficient of contraction than the ceramic materials. 4For example, the temperature of the container will drop from about 1800 F. to about l200 F. in less than a minute. However, the granular ceramic material loses heat very slowly partly because of its mass and partly because of its lower coefficient of heat transfer. Accordingly, the lateral walls of the container 12 apply a compressive load to the granular material. This compressive load is transferred through the granular material to the ceramic mold as represented by arrows 19 in FIGS. 3 and 4. Because the compressive load can be transmitted directly to external lateral wall surface 22 in FIG. 3, distortion or damage to the associated wall 20 has occurred prior to the present invention as shown in that FIG. 3.

In FIG. 4, compressive load represented by arrows 19 is shown to distort the glass cloth cushion 21 such as from a position shown in solid to one shown in phantom. This cushion absorbs the load preventing the transmission of the compressive load to the mold itself. In prior practice, the absence of a cushion 21 to cooperate with the external surfaces 22 of the mold, such as at wall portions 20, would allow the compressive load to bear directly on the mold. In large castings, as has been indicated before, compressive loads are developed sufficiently high to cause collapse of the mold wall resulting in a blockage of the flow of molten metal subsequently cast into the mold. Thus attempts`to cast articles by pouring molten metal into such blocked or damaged mold has resulted in castings ywith large voids completely through the casting wall.

In one specific example of the successful use of the present invention in its form shown in FIG. 4, a precision investment casting shell mold, including the usual gates, risers and sprues was made by the well known lost wax process. The ceramic used to coat the wax pattern was a siliceous material of the type known commercially as Nalcast material or Teco material.

The finished mold was placed on a bed of granular ceramic and vermiculite in an open top, circular metal container of a nickel-chromium alloy known as Inconel alloy with 0.125 thick walls and 1A inch reinforcing hoops and gussets and having an outside diameter of about 30 inches. The external lateral surfaces of the outer walls were then wrapped with glass ber cloth to a thickness sufficient to absorb loads resulting from differential expansion. In this case, a thickness of about 0.06 was satisfactory. The c-loth used in this example was of about the same weave and thickness as a normal burlap. This formed a space 30 in FIG. 4 between the glass cloth 21 and the wall surface 22 of the thin wall section `of the mold.

After wrapping the mold, the spaces between the wrapped mold and the container were filled with a granular material, including vermiculite, and granular aluminum silicate of the type commercially available as Mullite material, up to a level approximately as shown at 27 in FIG. 2. Then the container including the mold was placed in an air atmosphere preheat furnace where the temperature was stabilized in the range of about 1700-1800 F. The container was removed from the preheat furnace and placed in position in a vacuum casting chamber with a melting furnace from which molten metal was to be poured. The chamber was then evacuated to produce a vacuum of about -20 microns pressure and molten metal at about 2800" F. was cast into the mold. In this specic example, the metal cast was a nickel base alloy consisting essentially of, by weight, Cr, 3% Mo, 6% Cb and Ta, 0.6% Al, 0.7% Ti, 40.1 max percent carbon, 53% Ni, balance Fe.

This procedure including the present invention resulted in a sound casting. However, without the use of the glass cloth wrappings as a cushion to absorb the com pressive loads discussed before, castings having large breaks in the thin lateral wall portions resulted and were required to be scrapped.

In other examples, additional padding 21a in FIG. 5 of glass wool was placed between the glass cloth wrapping 2lb and the mold external surface 22 to Cushion further the side Walls and the other lateral mold portions. If desired, a single cushion coating 21 in FIG. 6 can be applied, for example by spraying a suitable flock-type material and binder onto the external surface of the mold.

Through the use of a cushion around external wall surfaces of molds for the production of relatively large castings including thin-walled portions, the precision investment casting of such articles has been made practical. Although this invention has been described in connection with specific examples and embodiments, it will be understood by those skilled in the art the variations and modications of which this invention is capable. It is intended by the appended claims to include al-l such variations and modifications.

What is claimed is:

1. An improved investment casting apparatus for use in casting relatively large thin-walled articles comprising, in combination,

a metallic mold container having lateral walls;

a relatively large investment casting ceramic shell mold including an inner ceramic wall portion and an outer ceramic wall portion having an external lateral wall mold surface, the mold being located within the mold container so that the external lateral wall mold surface is in spaced relationship with the lateral walls of the container;

a granular ceramic packing material against the inner wall portion and in the space between the outer wall portion and the lateral walls of the container; and

a cushion of a temperature resistant compressible material around the external lateral wall mold surface between such mold surface and the granular material.

2. The apparatus of claim 1 in which:

the ceramic shell mold has an outside diameter of at least about l5 inches; and

the cushion is in contact with at least a portion of the external lateral wall mold surface.

3. The apparatus of claim 2 in which the cushion comprises fibers.

4. An improved investment casting ceramic shell mold as described in claim 1 in which the cushion of a temperature resistant compressible material is secured with at least a portion of the external lateral wall mold surface.

5. The shell mold of claim 4 having an outside diameter of at least about 15 inches, and in which the `cushion is a woven material wrapped around the external lateral wall mold surface.

6. The mold of claim 5 in which the cushion comprises glass cloth.

References Cited UNITED STATES PATENTS 3,367,393 2/1968 Lenahan et al 164-361 X FOREIGN PATENTS 865,714 4/ 1961 Great Britain. 1,031,587 6/ 1966 Great Britain.

I. SPENCER OVERHOLSER, Primary Examiner ROBERT D. BALDWIN, Assistant Examiner U.S. Cl. X.R. 164-23 

